1. Field of the Invention
In general, the present invention relates to spintronics. In particular, the present invention relates to creating a spin polarization of virtually all of the electrons in nonmagnetic semiconductors at an arbitrary spin polarization current in ferromagnetic material and at a wide range of temperatures including room temperature.
2. Description of the Related Art
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Over the past decade, new ventures in solid state electronic devices based on both the electron density and spin of electrons has led to the development of a new field: spintronics. Spintronics is the manipulation of electron spin in solid state materials. Spintronics creates the possibilities for designing ultra-fast, low-power scalable devices and applications for quantum computing. Among the practical spintronic effects is a giant magnetoresistance (GMR) in magnetic multilayers and tunnel ferromagnet-insulator-ferromagnet (FM-I-FM) structures. Discovery of GMR in magnetic multilayers has quickly led to important applications in storage technology. GMR is a phenomenon in which a relatively small change in a magnetic field results in a large change in the resistance of the material. The phenomenon of a large tunnel magnetoresistance (TMR) of FM-I-FM structures is being studied by product development teams in many leading companies. TMR is typically observed in FM-I-FM structures made of two ferromagnetic layers of similar or different materials, separated by an insulating thin tunnel barrier I, with thickness ranging between 1.4-2 nm. The tunnel current through the structure may differ significantly depending on whether the magnetic moment is parallel (low resistance) or anti parallel (high resistance). For example, in ferromagnets such as Ni80Fe20, Co—Fe, and the like, resistance may differ by up to 50% at room temperature for parallel (low resistance) versus antiparallel (high resistance) moments on ferromagnetic electrodes.
Recently, studies have been made in regard to giant ballistic magnetoresistance of Ni nano-contacts. Ballistic magnetoresistance is observed in Ni and some other nanowires in which the typical cross-section of the nano-contacts of the nanowire is a few square nanometers. The transport in this case is through very short constriction and it is believed to be with conservation of electron momentum (ballistic transport). The change in the contact resistance can exceed 20-fold.
Of particular interest is injection of spin-polarized electrons into nonmagnetic semiconductors, because of the relatively large spin-coherence lifetime of electrons and the possibility of controlling the electron spin by external fields. The use of different ferromagnetic-semiconductor-ferromagnetic (FM-S-FM) heterostructures have recently been suggested, including those using an electric field, an external magnetic field, and a nanowire current. All the proposed spintronic devices are spin valves in which one of the ferromagnetic-semiconductor junctions works as a spin injector, and another one works as a spin polarizer (spin filter). Spin injection into nonmagnetic semiconductors (NS) holds promise both for the new generation of high-speed low-power electronic devices and quantum computing.
Relatively efficient spin injection in heterostructures with magnetic semiconductor as a spin source has been reported in {Refs. [9]}, the entire contents of which are expressly incorporated herein by reference. High enough spin injection from ferromagnets into nonmagnetic semiconductors has recently been demonstrated at low temperatures. However, the highest degree of spin polarization (the amount of electrons whose spin is coherent, or oriented the same) of injected electrons in nonmagnetic semiconductors, Pn, observed in all of existent works was less than 32% at low temperatures, and less than 10% at room temperatures. Thus far, all of the attempts to achieve higher spin polarization have faced fundamental difficulties.
The principal difficulty of the spin injection from a ferromagnetic (FM) into a nonmagnetic semiconductor is that a potential barrier (Schottky barrier) always arises in the semiconductor near the metal-semiconductor interface. Numerous experiments show that the barrier height Δ is determined by surface states forming on the interface, and is approximately (⅔) Eg, practically independently of the type of the metal. Eg is the energy band gap of the semiconductor, that is, the difference between the conduction band energy level EC and the valence band energy level EV. For example, for GaAs and Si the barrier height is equal to 0.5 eV-0.8 eV, with practically all metals, including Fe, Ni, and Co, and the barrier width, the length of the Schottky depleted layer lD, is relatively large (lD≈40 nm for doping concentration Nd≈1017 cm−3).
FIG. 1A illustrates a schematic of a conventional FM-S Schottky junction 100. As shown, the spin-injection junction 100 includes a semiconductor 110 and a ferromagnetic (FM) layer 120 above the semiconductor 110. The device 100 also includes electrodes 130 and 140 connected to the ferromagnetic layer 120 and the semiconductor 110. As will be described below, the Schottky barrier forms in such a way that the junction is very wide, which makes tunneling of electrons practically impossible.
FIG. 1B illustrates an energy band diagram of the conventional spin-injection device 100, illustrated in FIG. 1A. The barrier for electrons has a height Δ and width l (which is the thickness of the Schottky depletion layer).
The amount of spin injection from FM into NS materials is determined by the current in reverse direction through the Schottky barrier, minus bias voltage applied to the FM (electron flow is directed from FM to semiconductor, and the current is directed to opposite direction). This current is usually extremely small, mainly due to the relatively large Schottky depleted layer lD and Δ>>kBT, where kB is the Boltzmann constant and T is the device temperature. In the forward-biased FM-S (ferromagnetic semiconductor) Shottky junctions, a minus-bias voltage is applied to the semiconductor, and the current can reach a large value only at a bias voltage qV close to Δ, where q is the elementary charge.
Realization of an efficient spin polarization in nonmagnetic semiconductors (NS) due to such a thermoemission current is problematic for several reasons. First, electrons in FMs with an energy F+Δ are weakly spin polarized, where F is the Fermi level. Second, according to standard theory, the thermionic current through Schottky junctions depends solely on the parameters of the semiconductor and not on the parameters of the metallic ferromagnet {13}. Therefore, the current could formally be spin-polarized in Schottky contacts. Thus, the effective spin injection in the conventional FM-S Schottky junction 100 is impossible for all practical purposes.
It has been proposed to use an ultrathin heavily doped semiconductor layer (δ-doped layer) between the FM material and a nondegenerate nonmagnetic semiconductor to increase the spin injection at room temperature, as shown in FIG. 2A. This δ-doped layer of a thickness l+1 nm sharply reduces the thickness of the Schottky barrier, and increases its tunneling transparency. According to {14, “EFFICIENT NONLINEAR ROOM-TEMPERATURE SPIN INJECTION FROM FERROMAGNETS INTO SEMICONDUCTORS THROUGH A MODIFIED SCHOTTKY BARRIER”, V. V. Osipov and A. M. Bratkovsky, Phys. Rev. B 70, 205312 (2004); cond-mat/0307030 (2003).}, the entire contents of which are expressly incorporated herein by reference, the thickness of the δ-doped layer, l+, should be on the order of a typical tunneling length for the barrier l+l0 wherel0=(h2/8π2m*Δ)1/2  (1)And m* is the effective mass of electrons in the semiconductor δ-doped layer. Moreover, the bottom of conduction band, Ec0, in the semiconductor δ-doped layer and the nonmagnetic in equilibrium should be higher than the Fermi level, Ec0>F. The semiconductor has to be nondegenerate in whole semiconductor region including the δ-doped layer, as shown in FIG. 2B. It has been shown theoretically that spin polarization in the nonmagnetic semiconductor, Pn, both in the reverse-biased and forward-biased FM-S junction can approach that in the FM material at an electron energy of E≈Ec. In the reverse-biased FM-S junction, the predictable value of Pn can achieve a maximum, Pmax, at room temperatures when Ec corresponds to a spike of density of minority electron states in the FM material, as shown in FIG. 2B. Even a theoretical value of Pmax, however is substantially smaller than 1 at room temperatures and Pn=0 at low temperatures. In the forward-biased FM-S junction, Pmax is determined by spin polarization of current in the FM material which is substantially smaller than 1 (by about 30%-40%) {see [15]}.
Characteristics of all of spintronic devices improve dramatically with increase in the degree of the electron spin polarization, P.sub.n, and achieve to them, limited values when Pn=1 (100%). Moreover, a fundamental problem for quantum computing is to obtain an electron spin polarization in nonmagnetic semiconductors (NS) of P.sub.n=100% at very low temperatures, such as, T<1.degree. K.